Re: [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Alex Bligh <alex@alex.org.uk>]

--On 8 March 2013 12:54:16 +0000 George Dunlap 
<George.Dunlap@eu.citrix.com> wrote:

> On Fri, Mar 8, 2013 at 11:40 AM, Alex Bligh <alex@alex.org.uk> wrote:
>> Due to what is almost certainly a kernel bug, writes with
>> O_DIRECT may continue to reference the page after the write
>> has been marked as completed, particularly in the case of
>> TCP retransmit. In other scenarios, this "merely" risks
>> data corruption on the write, but with Xen pages from domU
>> are only transiently mapped into dom0's memory, resulting
>> in kernel panics when they are subsequently accessed.
>>
>> This brings PV devices in line with emulated devices. Removing
>> O_DIRECT is safe as barrier operations are now correctly passed
>> through.
>
> Not qualified to comment on the technical merits of the patch, but re
> the commit message:
>
> Acked-by: George Dunlap <george.dunlap@eu.citrix.com>

Any further thoughts on this one?

-- 
Alex Bligh




---------- Forwarded Message ----------
Date: 8 March 2013 11:40:44 +0000
From: Alex Bligh <alex@alex.org.uk>
To: xen-devel <xen-devel@lists.xen.org>, Stefano Stabellini 
<stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>, Alex Bligh <alex@alex.org.uk>, 
Ian Jackson <Ian.Jackson@eu.citrix.com>, Konrad Rzeszutek Wilk 
<konrad.wilk@oracle.com>, Jan Beulich <JBeulich@suse.com>, George Dunlap 
<George.Dunlap@eu.citrix.com>
Subject: [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default 
as it results in crashes.

Due to what is almost certainly a kernel bug, writes with
O_DIRECT may continue to reference the page after the write
has been marked as completed, particularly in the case of
TCP retransmit. In other scenarios, this "merely" risks
data corruption on the write, but with Xen pages from domU
are only transiently mapped into dom0's memory, resulting
in kernel panics when they are subsequently accessed.

This brings PV devices in line with emulated devices. Removing
O_DIRECT is safe as barrier operations are now correctly passed
through.

See:
  http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html
for more details.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
---
 hw/xen_disk.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/hw/xen_disk.c b/hw/xen_disk.c
index a402ac8..14f8723 100644
--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -603,7 +603,7 @@ static int blk_init(struct XenDevice *xendev)
     }

     /* read-only ? */
-    qflags = BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
+    qflags = BDRV_O_CACHE_WB | BDRV_O_NATIVE_AIO;
     if (strcmp(blkdev->mode, "w") == 0) {
         qflags |= BDRV_O_RDWR;
     } else {
-- 
1.7.4.1



---------- End Forwarded Message ----------



-- 
Alex Bligh